Instances exist, e.g. in the treatment of paper or other sheet material by vertically superposed calendering rollers, where it is important to maintain the axes of such rollers as parallel as possible to each other. Conventional means for checking on their parallelism, e.g. optically with the aid of a theodolite, are cumbersome and at times difficult to implement, as where parts of the roller pair are obscured by a machine frame supporting same.